Recently, small-sized and light-weighted electric/electronic apparatuses such as cellular phones, notebook computers, and camcorders have become widely developed and manufactured. These portable electric/electronic apparatuses have built-in battery packs in order to operate in any place where a separate power supply is not provided. The built-in battery pack includes at least one battery for outputting a predetermined level voltage to drive the portable electric/electronic apparatus for a predetermined time period.
Recently, these battery packs have begun employing rechargeable secondary batteries for economical usage. The secondary batteries may generally include a nickel-cadmium (Ni—Cd) battery, a nickel-hydrogen (Ni-MH) battery, or a lithium secondary battery, such as a lithium (Li) battery or a lithium ion (Li-ion) battery.
The operating voltage (3.6 V) of a lithium secondary battery is three times higher than those of a nickel-cadmium battery or a nickel-hydrogen battery widely used for the portable electronic apparatuses. The energy density (energy per unit weight) of the lithium secondary battery is larger than that of a nickel-cadmium battery or a nickel-hydrogen battery. Therefore, the usage of lithium secondary batteries has rapidly increased.
In a lithium secondary battery, a lithium oxide material is used as a positive electrode activation material, and a carbon material is used as a negative electrode activation material. In general, the lithium secondary batteries are classified as a liquid electrolyte battery or a polymer electrolyte battery, according to the type of electrolyte solution used. In addition, a battery using a liquid electrolyte is called a lithium ion battery, and a battery using a polymer electrolyte is called a lithium polymer battery. In addition, lithium secondary batteries can be manufactured in various shapes. Representative shapes of the lithium secondary batteries include a cylinder, a polygon, and a pouch.
Typically, a cylindrical lithium secondary battery includes an electrode assembly having a positive electrode plate coated with a positive electrode activation material, a negative electrode plate coated with a negative electrode activation material, and a separator interposed between the positive and negative electrode plates to prevent short-circuit therebetween. These components are substantially cylindrically wound, and a cylindrical case accommodates the electrode assembly. An electrolyte solution is injected into the cylindrical case to allow lithium ions to move.
A cylindrical lithium secondary battery is typically manufactured as follows.
Firstly, a positive electrode plate coated with a positive electrode activation material and connected to a positive electrode tap, a negative electrode plate coated with a negative electrode activation material and connected to a negative electrode tap, and a separator are stacked. Next, the stack is wound in a substantially cylindrical shape, so that a substantially cylindrical electrode assembly is formed.
Next, the substantially cylindrical electrode assembly is accommodated into a cylindrical case, a process for preventing the electrode assembly from being separated is performed, the electrolyte solution is injected into the cylindrical case, and a sealing process is performed, so that a cylindrical lithium secondary battery is formed.
In general, in a cylindrical lithium secondary battery, an empty space is formed at a central portion of the electrode assembly. This may pose a problem, however, in that the electrode assembly may be unwound and deformed due to the empty space.
In order to solve this problem, there has been proposed a method of inserting a predetermined core member into the central space of the electrode assembly as shown in FIG. 1. In general, the core member 100 is formed by winding a kind of plate to have a circular cross section. In addition, a portion of the core member is cut along the longitudinal direction thereof.
However, there is a problem in that the core member 100 can be easily deformed or broken by external pressure. When the core member 100 is deformed or broken, the separator of the electrode assembly may be broken. The breakage of the separator may cause a short-circuit between the positive and negative electrode plates, which may cause the cylindrical lithium secondary battery to break or explode.
In addition, when the core member 100 is deformed due to external pressure, particularly, a lateral pressure, the cross section of the pipe shape may be non-uniformly deformed. Therefore, there is a problem in that the separator may be seriously broken.